Normally this type of distance meter has a comparator in the measuring channel which performs a propagation time measurement between the transmitted and received signal pulses as is described in for example EP-A-475 326. The trigger point and delay in a comparator are always partly dependent on the input amplitude of the received signal pulse because the pulse is not square shaped but has a non-negligible rise time. Such an error should be eliminated.
If a pulse received from the target is detected in a comparator, then the signal-noise ratio for each pulse must be sufficiently high in order to eliminate false triggering by pure noise. This condition determines the range of the instrument. If the light source used is a laser or light diode then it must not be too powerful or give a too powerful radiation per time unit in accordance with the safety classifications for laser based instruments.
Different techniques have been used in order to perform an accurate propagation time measurement. A common method is to make a coarse determination through counting clock pulses from a high frequency clock. The resolution, however, is not sufficient with reasonable clock frequencies, wherefore the determination is complemented with an interpolation method. Such a distance measuring device is described in U.S. Pat. No. 4,620,700. This type of distance measuring device is adversely affected by non-linearities in the position determination.